Wireless systems, utilizing services such as mobile telecom, radio systems or wireless networks may use multiple antennas at the Base-Station (BS) to support multiple Mobile-Stations (MSs) at the same time and on the same frequency. The signal separation of the MSs is achieved in the spatial domain. Such a system is often referred to as a Multi-User (MU) Multiple-Input Multiple-Output (MIMO) system.
In this document an antenna is assumed to be a port, which is accessible for the transmission/reception of a wireless signal. This not necessarily needs to be a physical antenna. A physical antenna, might for example support two polarizations and therefore provide two accessible ports. In our notation these two ports are referred to as two antennas.
A key issue with MU-MIMO as well as other communication systems is the fairness concerning the resources allocated to different MSs. If a system was to maximize the total throughput, the BS would preferably serve MSs, which have a good channel quality, while discarding MSs with a bad channel quality. This near-far effect results in a non-uniform distribution of the available throughput over a communication cell.
A potential solution to this problem could be to increase the density of BSs in the system. Increasing the number of BS stations however becomes increasingly difficult since available sites are rare and costly. Relay Stations (RSs) are seen as a possibility to counter act the non-uniform distribution of system resources over the cell area, while avoiding the deployment of additional BSs. RSs have the advantage that they can be made small in size and do not require a back-haul connection. In addition, RSs may be used for extending the coverage of a communication system.
Relay based communication has been discussed in earlier works such as US 2007/0165581 A1, US 2008/0267111 A1, and US 2007/0190034 A1 These works however only treat the problem of communication between a single source (in this case the BS) and a single destination (in this case the MS) via RS(s). In US 7406060BB a MIMO based relay system is proposed, which focuses on the uplink of a communication system. Other related publications considering the transmission of signals from the BS to multiple MSs via RS(s), rely on the assumption that there is no direct signal path between the BS and the MS(s). Such documents are:    Tetsushi Abe, Hui Shi, Takahiro Asai, and Hitoshi Yoshino, “Relay techniques for mimo wireless networks with multiple source and destination pairs,” EURASIP J. Wire!. Commun. Netw., vol. 2006, no. 2, pp. 37-37, 2006.    A. Wittneben, “Coherent multiuser relaying with partial relay cooperation,” in IEEE Wireless Communications and Networking Conference, WCNC, 2006, vol. 2, pp. 1027-1033.    Lingfan Weng and R. D. Murch, “Multi-user MIMO Relay System with Selfinterference Cancellation,” in Wireless Communications and Networking Conference, 2007.WCNC 2007. IEEE, Kowloon, Mar. 11-15, 2007, pp. 958-962.    Taiwen Tang, R. W. Heath, Robert W. Heath, and Sunghyun Cho, “On Achievable Sum Rates of A Multiuser MIMO Relay Channel,” in Information Theory, 2006 IEEE International Symposium on, Seattle, Wash., July 2006, pp. 1026-1030.
Practical considerations such as the limited size of RSs generally require that RSs operate in a half-duplex mode. This half-duplex mode may be realized using Time-Division Duplex (TDD), i.e., a RS can receive in one time-slot and transmit in the next time-slot. Half-duplex operation of RSs in general results in an effective throughput-loss over the relay link of 50%, since the RS uses two time-slots to deliver its data to the receiver. During the second time-slot the BS even might be prevented from transmitting at all to avoid interference with the relay, which results in a further throughput loss.
Let us for example assume an area in a communication cell with bad signal quality and/or a high density of MSs. Given such an area the BS in a conventional communication system faces the problem of allocating resources to MSs in this area without wasting power and penalizing other MSs in the cell. If half-duplex RSs would be deployed to serve MSs in this area, system resources such as time-slots/frequency are blocked by the relay-link and the total system capacity would be decreased. Again other MSs are penalized.